Ethanol has been increasingly used as a transportation fuel, mostly blended with gasoline. In the US, most gasoline in the market contains 10% ethanol. High concentration blends (E85, 85% ethanol and 15% gasoline by volume) are sold in Brazil, Sweden, and in some states in the US. Other countries are planning to use ethanol.
Compared with gasoline, ethanol has superior anti-knocking properties and its heat of vaporization is about three times as high as gasoline. Both characteristics are highly desirable for direct injection spark ignition engine downsizing strategies which could significantly reduce fuel consumption of vehicles without compromising performance. When an ethanol/gasoline blend is burned in an internal combustion engine, oxygenates are produced as unburned fuel or partial oxidation products. Exhaust gas studies have found that ethanol and acetaldehyde are the main oxygenate species. Acetaldehyde is carcinogenic if inhaled. It is also an ozone precursor which contributes to smog formation. Ethanol in the atmosphere is further oxidized to acetaldehyde, which increases the atmospheric acetaldehyde inventory.
Higher ethanol content in fuel leads to more organic gas emission from an internal combustion engine. At present, the US EPA and CARB emission regulations for E85 requires speciated measurement of ethanol and carbonyls. The dominant carbonyl species is acetaldehyde. The CARB test procedure recommends water impinge sampling followed by gas chromatograph (GC) analysis for ethanol measurements. The carbonyl measurement is performed through acidified 2,4-dinitrophenylhydrazine cartridge sampling and high performance liquid chromatography (HPLC) analysis. These methods are sensitive to very low concentration and free of interference from other species, but require considerable manual handling and lengthy analysis time. Another method for measuring ethanol concentration involves the use of photoacoustic sensors (PAS), which has been approved by CARB as equivalent to the water impinger method. PAS utilizes a much simplified sampling method, and the result is available online, similar to conventional emission analyzers. However, compensation for ammonia and carbon dioxide is required to get an accurate ethanol concentration. Fourier Transform Infra Red Spectroscopy (FTIR) can also be used to detect ethanol and acetaldehyde, but is subject to interference from other species. In summary the challenge for ethanol and acetaldehyde measurements is to isolate these species for analysis from a complex mixture of exhaust gas, which may contain hundreds of components. Chromatography is an effective means to resolve the mixture, but the analytical procedures are relatively complex and time consuming. Attempting to analyze the mixture without separation is quicker but generally has cross interference problems.
In mass spectrometry, a gas mixture is admitted to a high vacuum chamber, where it is ionized. Ions are then separated by their mass-to-charge ratio using various means depending on the type of mass analyzer, e.g. quadrupole, magnetic sector, or time-of-flight. Separating exhaust gas species by mass does not guarantee freedom from interference. For example, ethanol and nitrogen dioxide both have a nominal atomic mass of 46. Similarly, carbon dioxide and acetaldehyde both have an atomic mass of 44. Moreover, during the ionization process a gas molecule can split into smaller fragments, adding another source of interference. To minimize the effect of fragment ions, most mass spectrometers used in exhaust gas analysis utilize chemical ionization. Chemical ionization is a low energy process, which yields fewer fragments. Some methods of detecting ethanol and acetaldehyde have used three different chemical ionization levels, chosen to avoid ionizing interfering species. Nevertheless, ethanol still interfered with acetaldehyde in these methods, requiring compensation.
Thus, there remains a need to develop mass spectrometric methods of detecting ethanol, acetaldehyde, and other components of exhaust gases without interference from other chemical species.